/Engine/Source/Runtime/AppFramework/Public/Widgets/Colors/SColorThemes.h
Panels arrange their children in a space described by the AllottedGeometry parameter. The results of the arrangement should be returned by appending aFArrangedWidgetpair for every child widget.
virtual void OnArrangeChildren ( const [FGeometry](API\Runtime\SlateCore\Layout\FGeometry) & AllottedGeometry, [FArrangedChildren](API\Runtime\SlateCore\Layout\FArrangedChildren) & ArrangedChildren ) const